Murty / Yeh / Ranganathan High-Entropy Alloys

1. Auflage 2014
ISBN: 978-0-12-800526-2
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 218 Seiten

This book provides a complete review of the current state of the art in the field of high entropy alloys (HEA). The conventional approach to alloy design is to select one principal element and add elements to it in minor quantities in order to improve the properties. In 2004, Professor J.W. Yeh and his group first reported a new approach to alloy design, which involved mixing elements in equiatomic or near-equiatomic proportions, to form multi-component alloys with no single principal element. These alloys are expected to have high configurational entropy and hence were termed as 'high entropy alloys.' HEAs have a broad range of structures and properties, and may find applications in structural, electrical, magnetic, high-temperature, wear-resistant, corrosion-resistant, and oxidation-resistant components. Due to their unique properties, high entropy alloys have attracted considerable attention from both academics and technologists. This book presents the fundamental knowledge present in the field, the spectrum of various alloy systems and their characteristics studied to date, current key focus areas, and the future scope of the field in terms of research and technological applications. - Encompasses the synthesis and phase formation of high entropy alloys - Covers design of HEAs based on thermodynamic criteria - Discusses the structural and functional properties of HEAs - Provides a comparison of HEAs with other multicomponent systems like intermetallics and bulk metallic glasses

Dr. B.S. Murty is a Professor, Department of Metallurgical and Materials Engineering, IIT Madras, Chennai, India. He has pioneered the synthesis of nanocrystalline materials by mechanical alloying route. His other important contributions are in the field of bulk metallic glasses and in-situ composites. He has co-authrored a Text Book on Nanoscience and Nanotechnology. He has also made significant contributions to the field of high-entropy alloys.

Autoren/Hrsg.

Murty, B. S.

Yeh, Jien-Wei

Ranganathan, S.

Leseproben

Chapter 2 High-Entropy Alloys
Basic Concepts
High-entropy alloys (HEAs) have emerged as an important class of materials in the last one decade with immense potential both at the level of fundamental understanding and also in terms of applications. This chapter deals with the basic concepts that underpin this new class of alloys. It starts with the definition of HEAs and how they differ from other multicomponent alloys such as Ni-base superalloys. Then it gives a classification of alloys in terms of the number of components. A notation that is commonly used to represent composition in these alloys, particularly the nonequiatomic alloys with alloying additions, is brought out. Finally, four core effects of HEAs are elucidated, namely, high-entropy, severe lattice distortion, sluggish diffusion, and cocktail effects. Keywords
Classification; definition; notation; core effects; severe lattice distortion; sluggish diffusion; cocktail effect 2.1 Introduction
As the combinations of composition and processes for producing HEAs are numerous and as each HEA has its own microstructure and properties to be identified and understood, the research work is truly limitless (Zhang et al., 2014). It becomes very important to consider the basic concepts relating to HEAs at the very beginning, including the origin of high entropy, classification, definition, composition notation, and the four core effects of HEAs. 2.2 Classification of Phase Diagrams and Alloy Systems
Phase diagrams and the systems they describe are often classified and named after the number, in Latin, of components in the system. They can be christened based on their number of components as unary, binary, ternary, quaternary, quinary, sexinary, septenary, octonary, nonary, and denary from single- to 10-component alloys, respectively. It is suggested that this nomenclature recommended by ASM Handbook Volume 3 (1992) may be followed. This volume acquires renewed interest as phase diagrams are the beginning of wisdom, as observed by Hume-Rothery. There are very few binary engineering alloys that are commercially in use and most alloys are actually multicomponent alloys. However, most of these multicomponent alloys are still based on one principal element and are named after the principal elements, such as Fe-, Al-, Cu-, Ni-, and Ti-based alloys. These alloys may consist of solid solutions and/or intermetallic compounds (crystalline or quasicrystalline). These often include interstitial compounds, known as Hagg phases. Under nonequilibrium conditions some of the alloys form metallic glasses. Cantor (2007), has recently shown that the total number of possible alloys (N) that can be formed with C number of components, when each alloy differs in composition by x%, can be written as N=(100/x)C-1. If we consider 60 different elements (excluding those that are too radioactive, toxic, rare, and/or otherwise difficult to use) in the periodic table to combine with each other to form alloys, the total number of possible alloys turns out to be about 10177, if each alloy differs in composition by 0.1%. Even if we reduce the number of elements to 40 that can be used to make the alloys and considering that each alloy differs in composition by 1%, the total number of possible alloys turns out to be 1078, which is an astronomical number, considering that there are only 1066 atoms in the galaxy (Cantor, 2007). The mutual solubility between solvent and solute components in a binary alloy system could be judged by Hume-Rothery rules, namely, crystal structure, atomic size difference, valence, and electronegativity. In fact, all these factors also influence the interaction between different elements and make the enthalpy of mixing either negative (attractive interaction leading to ordering and the formation of intermetallic compounds), positive (repulsive interaction leading to clustering and segregation), or near zero (leading to the formation of disordered solid solutions). The competition between enthalpy of mixing and entropy of mixing further affects the solubility between two components. When solubility is limited, terminal solid solutions based on each component can be obtained in the phase diagram. When a solid solution forms at all compositions, it is called an isomorphous system. But continuous solid solutions in binary alloy system are not common because the conditions for its formation are very strict to fulfill. Available binary alloy phase diagrams (Massalski, 2001) indicate that the total number of isomorphous binary and ternary systems is only 153 and 248, respectively. While such terminal solutions are well known in binary, ternary and quaternary alloy systems, it is curious to know whether we can obtain solid solution phases in the center of higher order phase diagrams or not. In fact, formation of such solid solutions in the center of the phase diagram has not been explored much (Figure 2.1). In a significant deviation from traditional ways of making alloys, Cantor et al. (2004) and Yeh et al. (2004b) independently came up with the idea of preparing equiatomic or near-equiatomic multicomponent alloys. Yeh et al. (2004b) popularized these alloys as “HEAs” by pointing out the known thermodynamic fact that configurational entropy of a binary alloy (?Sconf =-R(XA ln XA+XB ln XB)) is a maximum when the elements are in equiatomic proportions (Figure 2.2) and that the maximum configurational entropy (?Sconf,max=RlnN) in any system increases with increasing number of elements (N) in the system (Figure 2.3). He emphasized that the high mixing entropy (in this book “configurational entropy” is referred to as “mixing entropy/entropy of mixing” in some places to be in tune with literature. However, both these terms should be treated as same) in HEAs would have a profound effect on the constituent phases, kinetics of phase formation, lattice strain, and thus properties. In particular, it enhances the mutual solubility between constituent components and leads to simpler phases and microstructure not expected before. This simplification originating from high-entropy effect is thus very important in such multicomponent alloys. Therefore, many possible new materials, new phenomena, new theories, and new applications are foreseen in the twenty-first century (Yeh et al., 2004b).
Figure 2.1 Schematic ternary and quaternary systems, showing regions of the phase diagram which are relatively well known (green) near the corners, and relatively less well known (white) near the center. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this book.) Adapted from Cantor (2007).
Figure 2.2 Configurational entropy reaching a maximum at the equiatomic composition in a binary system.
Figure 2.3 Entropy of mixing as a function of the number of elements for equiatomic alloys in the random solution state. This concept has opened up flood gates of research activity and over the past decade about 400 papers have been published on HEAs, out of which about 100 are from Yeh’s group itself. Though Yeh et al. (2004b) originally proposed that due to the large mixing entropy, HEAs tend to form solid solutions including disordered and partially ordered solid solutions intense research in recent years has been done on the rules or criteria using parameters including configurational entropy, enthalpy of mixing and the atomic size difference between different elements to predict the nature of phase formed in the alloy. Furthermore, computational thermodynamics methods such as CALPHAD have been used for predicting phase diagrams for HEAs in a more direct manner. In addition to studying the basic phases and microstructure of different equiatomic multicomponent alloys ranging from five to twenty components (Cantor et al., 2004), Cantor et al. (2002) introduced a second strategy of equiatomic substitution of elements in binary alloys, wherein various similar elements are substituted in equal proportions in a binary system. This research was motivated by a desire to determine favorable compositions for the formation of metallic glasses (Figure 2.4). For example, in Zr50Cu50 alloy, he substituted 75% of Zr with Ti, Hf and Nb such that all four elements are in equiatomic proportions. Likewise, Cu was substituted partially with Ag and Ni so that all the three elements are in equiatomic proportions to arrive at a final septenary alloy composition of (Ti0.25Zr0.25Hf0.25Nb0.25)50(Cu0.33Ag0.33Ni0.33)50. Even though this alloy has seven components, it can still be treated as a pseudobinary system and Cantor and his group have demonstrated good glass forming ability in a number of such pseudobinary alloys. They have also added Al to the alloy to form pseudoternary alloys. Of course such substitution of like elements in place of the two components need not be in equal proportions and several metallic glasses are known with such nature, for example, an Fe80B20 glassy alloy, in which Fe can be substituted by a number of transition elements such as Cr, Co, and Ni and B can be substituted by other metalloids such as Si and C. One can demonstrate similar substitution even in case of intermetallic compounds (NiAl, wherein Ni can be substituted by Fe, Co) and even in oxides such BaTiO3, wherein Ba...